Very high dynamic-range switched capacitor ADC with large input impedance for applications tolerating increased distortion and noise at large input signal levels

Abstract

A modulator is configured to respond to input swings by providing a feedback voltage via a feedback path to compromise an increase in noise and distortion power with increasing signal power at signal levels exceeding a predetermined threshold. A digital-to-analog converter (DAC) generates a feedback voltage with a resistor string biased with a given current and switches as a function of an input value to mitigate the voltage swing at a summing node.

What is claimed is: 
     
       1. A modulator comprising:
 an analog input configured to receive an analog input voltage; 
 a modulator path, comprising a modulated digital output, configured to generate a modulated digital output signal at the modulator digital output; and 
 a feedback path, coupled to the modulated digital output, comprising a digital-to-analog converter configured to generate a floating voltage as a function of the modulated digital output signal. 
 
     
     
       2. The modulator of  claim 1 , further comprising:
 a summing node configured to receive the analog input voltage and generate a voltage to the modulator path as a function of the analog input voltage of the analog input and the floating voltage of the feedback path. 
 
     
     
       3. The modulator of  claim 1 , wherein the digital to analog converter is configured to compromise analog noise and total harmonic distortion in response to the analog input voltage exceeding a predetermined threshold. 
     
     
       4. The modulator of  claim 1 , wherein the digital to analog converter further comprises:
 a resistor string coupled to the analog input comprising a set of resistors coupled to one another; 
 a first current source, coupled to a first end of the resistor string, configured to generate a first current; 
 a second current source, coupled to a second end of the resistor string, configured to generate a second current; and 
 configured to provide a set of voltage levels to the modulator path. 
 
     
     
       5. The modulator of  claim 1 , wherein the analog input is coupled to a resistor string at a center tap located at an approximate center of the resistor string. 
     
     
       6. The modulator of  claim 1 , further comprising:
 a resistor string, coupled to the analog input, configured to generate a set of voltage levels to the modulator path based on a state of a plurality of switches that selectively provide the set of voltage levels to the modulator path at an output tap of the digital to analog converter. 
 
     
     
       7. The modulator of  claim 6 , further comprising:
 a modulator code component configured to generate a switching code that controls the state of the plurality of switches and select which output tap of the resistor string to provide the floating voltage to the modulator path from the digital to analog converter, 
 wherein the plurality of switches is configured to provide an offset voltage to the output tap to modify an input voltage of the analog input as a function of the state of the plurality of switches, the first current of the first current source or the second current of the second current source, and a total resistance between a center tap of the resistor string and the selected output tap. 
 
     
     
       8. The modulator of  claim 6 , wherein the digital to analog converter further comprises selective combinations of fixed current sources connected to a first end and a second end of the resistor string and adjustable current sources connected to a top and a bottom of at least two center resistors of the resistor string that are driven by the input signal and are controlled by a modulator code to mitigate non-idealities, or mismatches between the adjustable current sources leading to improved THD at a cost of a higher power consumption. 
     
     
       9. The modulator of  claim 1 , wherein the digital to analog converter comprises a constant power supply current, and is further configured to generate an analog noise contribution that increases in response to the signal value of the analog input voltage at the analog input satisfying a predetermined threshold. 
     
     
       10. The modulator of  claim 1 , wherein the modulator path comprises:
 a continuous time buffer coupled to an output of a summing node that combines the floating voltage and the analog input voltage of the analog input; 
 a switched capacitor loop filter configured to receive an output of the continuous time buffer and load the output of the continuous time buffer with a sampling capacitance at a first integrator to generate a filtered output; and 
 a multi-bit analog to digital converter configured to quantize an analog output of the switched capacitor loop filter and generate a multi-bit digital output. 
 
     
     
       11. An audio system comprising:
 an analog input configured to receive an analog input voltage; 
 a modulator component, comprising a modulated digital output, configured to generate the modulated digital output; and 
 a feedback component, coupled to the modulated digital output, comprising a digital to analog converter configured to generate a floating voltage and moderate a voltage swing from the analog input voltage at the analog input to the modulator path with the floating voltage. 
 
     
     
       12. The audio system of  claim 11 , wherein the digital to analog converter is configured to modify a resistance as a function of the analog input voltage at the analog input to adjust the floating voltage. 
     
     
       13. The audio system of  claim 11 , further comprising:
 a acoustic sensor configured to detect an audio signal as a function of an electrical signal generated from a membrane change; 
 a buffer, with gain or attenuation or unity gain, comprising an input impedance that facilitates communication of the electrical signal to the modulator component; and 
 a high-voltage charge pump configured to supply a bias voltage to the acoustic sensor; and 
 a voltage multiplier configured to supply the buffer and the digital to analog converter of the feedback path. 
 
     
     
       14. The audio system of  claim 11 , wherein the digital to analog converter is configured to compromise an increase in a noise power or a total harmonic distortion in response to the analog input voltage satisfying a predetermined threshold. 
     
     
       15. The audio system of  claim 11 , wherein the modulator component comprises:
 a switched capacitor loop filter configured to load the floating voltage with a sampling capacitance to generate a filtered output; and 
 a multi-bit analog to digital converter configured to quantize an analog output of the switched capacitor loop filter and generate a multi-bit digital output. 
 
     
     
       16. The audio system of  claim 11 , wherein the feedback component further comprises:
 a resistor string comprising a set of resistors configured to selectively provide one or more voltage levels to the modulator path as a function of the analog input voltage; and 
 a summing node configured to receive the analog input voltage and generate a voltage as a function of the analog input voltage of the analog input and the floating voltage of the feedback path. 
 
     
     
       17. The audio system of  claim 11 , wherein the feedback component further comprises:
 a resistor string, coupled to the analog input, configured to selectively provide one or more voltage levels to the modulator path as a function of the analog input voltage; 
 a first current source, coupled to a first end of the resistor string, configured to generate a source current to the resistor string; and 
 a second current source, coupled to a second end of the resistor string, configured to generate a sink current from the resistor string. 
 
     
     
       18. The audio system of  claim 17 , further comprising:
 an modulator code component configured to control one or more switches of the resistor string to facilitate the one or more voltage levels to provide an offset voltage to the summing node to modify the analog input voltage of the analog input. 
 
     
     
       19. The audio system of  claim 11 , wherein the feedback component is further configured to moderate the voltage swing from the analog input voltage at the analog input to the modulator path with the floating voltage and is compromising a noise and distortion power in response to the analog input voltage satisfying a voltage threshold. 
     
     
       20. A method comprising:
 receiving, at an analog input of a modulator, an analog signal derived from a sensor; 
 modulating the analog signal in a modulator path to generate a digital output signal at a digital output; 
 generating a floating voltage via a digital to analog converter in a feedback path to the modulator path as a function of the digital output; and 
 modifying the floating voltage generated by the digital to analog converter in response to a change in a summing signal of a summing node coupled to the modulator path. 
 
     
     
       21. The method of  claim 20 , further comprising:
 generating a negative feedback at the summing node of the modulator path in response to the analog signal. 
 
     
     
       22. The method of  claim 20 , further comprising:
 controlling a voltage at the summing node that receives the analog signal by moderating the floating voltage in response to a predetermined threshold being satisfied and compromising to obtain a noise power with a signal power. 
 
     
     
       23. The method of  claim 20 , further comprising:
 selecting a set of resistors of a resistor string by controlling a set of switches in response to an output code of a modulator code component; and 
 generating the output code to operate the set of switches as a function of a summing voltage at the summing node. 
 
     
     
       24. The method of  claim 20 , further comprising:
 filtering the modulator path with a loop filter comprising a plurality of integrators; and 
 quantizing an analog output of the loop filter with an analog to digital converter with multiple bits concurrently in the modulator path to generate the digital output. 
 
     
     
       25. The method of  claim 20 , further comprising:
 modifying a resistance of the digital to analog converter to adjust the floating voltage as a function of a change in the digital output.